The present invention relates generally to LED driver circuits. More particularly, the present invention relates to non-isolated LED drivers with circuitry designed to control the voltage between an LED load and earth ground during input surge conditions.
Generally stated, non-isolated lighting circuits such as LED drivers may be desirable in many applications at least because they are smaller in size, lower cost and provide a higher efficiency as compared, for example, to isolated LED drivers.
Referring to FIG. 1, one example of a topology of a light fixture implementing a non-isolated LED lighting circuit or driver 10 is shown. V_in_AC is the AC input voltage source. An EMI filtering circuit includes L_common and L_diff as inductive elements and C_x is an EMI filter capacitor. C1 is a high frequency filter capacitor. Diodes D1-D4 form a diode bridge that rectifies the AC input from the EMI filtering circuit into a DC voltage. A non-isolated DC-DC converter 12 is used to control the LED current. The rectifier formed by diodes D1-D4 and the DC-DC converter 12 are coupled to circuit ground GND_main.
A surge protection circuit includes two clamping devices such as Metal-Oxide Varistors (MOV), which are used to protect the lighting circuit 10 on its input end from surge damage. A first clamping device MOV1 is connected between line and neutral inputs to clamp the input voltage to a certain value when a high voltage surge happens between the line and neutral inputs. A second clamping device MOV2 is connected between earth ground and neutral to clamp the input voltage between the earth ground, neutral and line inputs when the high voltage surge happens between line-neutral-earth ground.
A light source such as an LED load 14 is connected to receive an output current from the non-isolated DC-DC converter 12. Typically the chassis 16 of the LED load 14 will be grounded to earth 18, particularly for applications such as an outdoor LED lighting fixture. Between the LED load 14 and earth ground 18 there is accordingly an electrically equivalent tiny capacitance.
Therefore, when a surge voltage appears across line or neutral inputs and earth ground, all of the voltage as clamped by the second clamping device MOV2 will be forced across the LED body to earth ground. As shown in the example of FIG. 1, the clamped surge voltage is represented by V_surge. Because the clamping device MOV2 may typically be clamped to a value of at least 1 kV for 120-480 volt (V) input applications, the voltage across the LED load body and chassis will accordingly be at least 1 kV when such a surge occurs. A voltage of this magnitude can easily damage the LED engine during input surge conditions such as, for example, may occur from a lightning surge. This is a destructive drawback that limits the practical application of non-isolated DC-DC converter in LED drivers, particularly outdoor applications of drivers.
An ideal MOV or TVS (Transient Voltage Suppressor) could be connected between the LED input and chassis earth ground to limit the voltage across these components when a surge happens. However, each of these solutions presents problems of their own.
The normal operating RMS voltage of an MOV is typically two or three times lower than the clamping voltage. As a result, to set a clamping voltage of an MOV to a sufficiently small value, i.e. 500V, the normal operating voltage would be 100V-200V, which may be lower than the input voltage in certain applications. If the normal operating voltage of a MOV is less than the input voltage of the LED driver, the leakage current of a MOV may be large enough to overheat the MOV and cause it to fail during a normal operating condition.
A TVS can have the same clamping voltage and normal operating voltage. But for a high voltage TVS, the surge current capability is very limited, in fact providing much less than that offered by the MOV. Therefore, a TVS cannot be reasonably implemented for this type of protection circuit either.
It would therefore be desirable to provide a surge protection circuit which could reliably clamp the voltage between an LED load and an associated chassis ground to a low value, such that non-isolated DC-DC converters can be effectively implemented as a topology for LED driver applications.